Zinc plating



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United States Patent ZINC PLATING William H. Safranek and Hugh R. Miller, Columbus, Ohio, assignors, by mesne assignments, to American Serial-N0. 681,906 "S'Claims. "(Cl.'"204'55) This invention relates to zinc electroplating, and particularly to a method and compositionsfor electroplating bright, leveling zinc.

One object ofthe invention is to,v provide a method for eliminating orreducing the costs of polishing and boiling of die castings prior to electroplating with copper, nickel, and chromium. Heretofore, zinc die castings had to be polished and buffed by costly mechanical procedures to eliminate scratches, pits, cold shut, and other surface irregularities to make their surfaces sufficiently smooth for producing a mirrorlike finish by plating with copper, nickel, and chromium. Although "several leveling-type copper and nickel plating processes are employed commercially, they do not result in a satisfactory mirrorlike appearance unless the .die cast surfaces :are first smoothened mechanically. xHowever, by 'zinc plating with the process of.this=invention,.-asmooth bright finish can be obtainedon zincdie castings without-the needforpolishing or butting them. When the zinc-plated die castings are subsequently electroplated with leveling copper and nickel, and finally withchromium, an excellent'niirrorlike appearance is obtained.

Another object is to provide a methodfforimproving the corrosion'resistance-of electroplated zinc-die castings. Ithas been established that sulface poresin die cast surfaces seldomaare removed completely'by-mechanicalpolishing or bufling, and ithas been established further that such pores, which usually are neither filled nor "bridged by :copper, nickel, and chromium .plating,.are.the foca1 points for corrosion when electroplated 'diecastings'are exposed outdoors. However, .by zincplating according to this invention, surface pores are filled or bridged with zinc plate. Copper, nickel, and .chromium then canbe electroplated with no discontinuities -or pits, therebyimproving the corrosion resistance of the electroplated die castings.

A further object of this invention is to provide a-method for improving the appearance of 'zinc-plated'steel'parts. Although several processes are employed commercially for electroplating brightzzinc, they have little or'noleveling power and, as a result, steel partsstamped from hot rolled steel and other ,parts fabricated from commercial cold rolled steel with-a No. 1 or -No. 2 finish havean inferior, frosty or hazy-appearance 'after'zinc'platingby prior artprccesses. app aran of suchparts, how; ever, is improved greatly by zinc plating according to the present invention. Surfaces are more .nearly mirrorlike and irregularities :such as .scratchesand pits are covered or hidden, .becauseof the exceptional "leveling, power of the zinc plate .depositediby the present process.

Other objects and advantagesof thepresentinvention are apparent from the disclosure herein.

It has been found in the presentinvention that'cer'tain critical ranges, of composition .and operating conditions are necessary to provide optimumpolishing of scratches and'leveling, especially pore filling.

Table I Ibelow lists preferred and useful ranges of bath compositions andbperating conditions according. to the presentinvention.

, 2,905,603 Patented Sept. 22, 1959 TABLE r Bath composition Preferred I Usiul concentration concentration range mange Zinc sulfate, ZnSOflHiO g./L 230 to 250' 150-100. Ammonium chloride g./l 10 to 20 Orgtlggalup Aluminum sulfate, Alz(SOi)a.18Hi0 gI/l I 25to'-35 I Do. Glycyrrhizimammoniated g./l 1.0033 0 0.5to;7.0. Glycerin g./l- 5to7 o titlgnal'up o Tergitol 08, wetting agent percent by volume" Low-1.5 Qigtignfial up 5 l o pH 3.1 to 3.7 2.0 to 4.0. Boric Acid-. rOptional up to 45.g./l.

Op'eratingconditions Preferred Useful Range of ratios range range Periodic current reversal cycle: V Forward seconds 14-18 4-60 For. 15 Reverse do 4-6 1-45 to Cathode currentdensity: I

Forward amp./sq.ft.- 150-250 -500 For. 3 Reverse am s .tt 200-400 30-800 P- Temperature R. 130-155 90-180 "I-ergitol08=ls a 38 percent solution of sodium sulfate derivative of 2-ethyl hGXBHOI-IUJHDOH CHtS 04 Na.

Other typical conditions Agitation Work .bar'3060 c.'p.m., 1 to 4 inch--stroke Air-sufilcien't to raise the solution level about one inch. Anode current density-30 to 60 amp/sq. ft. Voltage. (between cathode and anode) Direct, 5 v 0 Reverse, 10 v 5-15 Plating rate-0.002 to 0.004 inch/hour. Time15=60 minutes.

Thickness of deposit-0.0005 to 0.004.

. arebuifers. Theammoniated glycyrrhiz in'is a'brig'htening agent. The glycerin increases the solubility of the glycyrrhizin. The wetting agent prevents pitting.

Any other ammonium halide may be used inpla'ce of the ammonium chloride listed in Table 'I. 'When the halide -is omitted a slightly higher'voltage maybe-required. "Other buffers may be substituted for the alusulfate or boric "acid, or both, if desired. When buffers are 'not'usetLthe pH should be checked'aud'adjusted regularly. Other wetting agents can also be substituted. The wetting agent can be omitted if the bath is maintained in a substantially pure condition, freefrom oil, .grease, andmetallic'impurities such as-copper, lead, and cadmium. The glycerin "increases the solubilityof the 'glycyrrhizinin thebath. Other solubility improvers such as high boiling alcohols can be substituted *desired. The glycerin or'substitute can be "omitted, 21 resulting slight "reduction of g lycyrrh'izin solubility.

The essentials of the bath composition are '(a) zinc sulfate in the'range of about to 400 grams perliter, preferably about 230m 250 grams per liter; (b) ammoniated glycyrrhizin in the 'rangeof about 0.5 to 7.0 gramsper liter,preferably about 1.0 to 3.0 gramsperliter; and acid concentration such as to provide a pH in the range ofabout 220-to 4.0, preferably about 3.1 to 3.7.

Table IIbelow lis'ts'seven examples ofplatingbaths and conditions in accordance with the present invention.

TABLE II Emomples of conditions used for plating leveling bright zinc Example No- 1 Supplemental constituent, g.ll 1 7 Concentration of ammoniated glycyrrhlzin, g./L 6 Periodic current reversal cycle:

Forward, er-nmi 16 Reverse, seconds 5 Current density:

Forward, amp/sq. ft Reverse, ampJsq. ft Tlemperature, F P ating time, minutes 30 Thickness of plate, inch. Plating rate, inches/hour... Appearance of plate cam III IV V 1 Glycerin. Glycer n plus 45 g./l. boric acid. Smooth bright.

Each bath contained:

240 g.[1. zinc sulfate (ZnSO4.7H,O)

30 grll. aluminum sulfate (AJZ(SOA)3-18H10) g.ll. ammonium chloride 0.5 to 1.5 percent, by volume Tergitol 08 (Tergitol 08 is the sodium sulfate derivative of 2-ethylhexanol) pH usually 3.4 to 3.8, but sometimes 2.5 to 3.1

tiIlhe plating bath was continuously filtered to remove insoluble parc es.

Bath voltages generally were: 5 to 7 volts for the forward part of the cycle 7 to 14 volts for the reverse part of the cycle All baths were agitated with air, and the cathode bar was moved back and forth from 30 to 60 c.p.m. with a 1-, 2-, or 4-inch stroke.

Cathode eflicienoy (direct Between 95 and 100 percent.

plating period).

Anodes High-purity zinc in cotton or nylon bags (to keep anode particles away from cathode).

Racks Copper, covered with masking tape and four coats of stop-oil lacquer to prevent tree formation.

b Surfaces were smoothened as follows:

Percent Before plating, R.M.S., mlcroinches leveling TABLE III Leveling data on zinc plating steel panels Surface roughness Surface roughness before after zinc plating, plating, R.M.S., R.M.S., mloroinches microinches Percent Thickness of zinc plate,

leveling mlllhnches s Zinc sulfate concentration was increased to 360 g./1.

Smooth, bright plates were obtained in all of the above examples. In Example II profilomcter measurements were made to measure the leveling obtained with three difiercnt surfaces as indicated in footnote (b) above. The profilometer measures the root mean square (R.M.S.) of the maximum deviations on the surface from a perfectly smooth plane. The leveling obtained is the reduction in percent of the profilometcr reading after the surface has been plated. The plates obtained with the present invention are noticeably smoother and brighter than those obtained by prior methods, and the leveling obtained on a given surface is substantially increased by the present invention.

The visible results of scratch leveling obtained by the present process were demonstrated on a flat surface of a zinc die casting that had been mechanically polished with abrasive papers to provide areas of fine scratches (using mctallographic paper with about 320-grit abrasiveland'arcas of coarse scratches (using 180-grit abrasive). The areas of fine scratches were completely leveled by plating with Zinc according to the present invention and adding thin coatings of electroplated copper, nickel, and chromium in a conventional manner. The reflection of images from these areas was free from distortion. The areas of coarse scratches, which were plated along with the other areas, were substantially leveled. Only a slight distortion was apparent in the image reflection f flle ss wa I r What is claimed is:

l. A method of bright, leveling zinc electroplating comprising plating zinc upon a conductive cathode from a zinc solution consisting essentially of about to 400 grams per liter of zinc suliate, and about 0.5 to 7.0 grams per liter of ammoniated glycyrrhizin, said solution having a pH from about 2.0 to 4.0; providing the plating by a periodic current reversal cycle alternating between forward current for about 4 to 60 seconds. at a cathode current density of about 50 to 500 ampcrcs per square foot and reverse current for about 1 to 45 seconds at a cathode current density of about 30 to 809 ampcres' per square foot, while maintaining the temperature of said solution between about 90 to 180 F.

2. The method of claim 1 in which the ratio of forward current time to reverse current time in said periodic current reversal cycle is about 1.3 to 15, and the ratio of forward cathode current density to reverse cathode current density is about 0.3 to 3.0.

3. A method of bright, leveling zinc electroplating comprising plating zinc upon a conductive cathode from a zinc solution consisting essentially of about 230 to 250 grams per liter of zinc sulfate, and about 1.0 to 3.0 grams per liter of ammoniated glycyrrhizin, said solution having a pH from about 3.1 to 3.7; providing the plating by a periodic current reversal cycle alternating between forward current for about 14 to 18 seconds at a cathode current density of about 150 to 250 amperes per square foot and reverse current for about 4 to 6 seconds at a cathode current density of about 200 to 400 amperes per square foot, while maintaining the temperature of said solution between about 130 to 155 F.

4. A composition of matter for bright, leveling zinc electroplating consisting of an aqueous solution c0nsisting essentially of about 150 to 400 grams per liter of zinc sulfate, and about 0.5 to 7.0 grams per liter of ammoniated glycyrrhizin, said solution having a pH from about 2.0 to 4.0.

5. A composition of matter for bright, leveling zinc electroplating consisting of an aqueous solution consisting essentially of about 230 to 250 grams per liter of zinc sulfate, and about 1.0 to 3.0 grams per liter of ammoniated glycyrrhizin, said solution having a pH from about 3.1 to 3.7.

References Cited in the file of this patent UNITED STATES PATENTS 2,250,842 Roberts July 29, 1941 2,451,341 Jernstedt Oct. 12, 1948 2,651,610 Chester Sept. 8, 1953 

1. A METHOD OF BRIGHT, LEVELING ZINC ELECTROPLATING COMPRISING PLATING ZINC UPON A CONDUCTIVE CATHODE FROM A ZINC SOLUTION CONSISTING ESSENTIALLY OF ABOUT 150 TO 400 GRAMS PER LITER OF ZINC SULTATE, AND ABOUT 0.5 TO 7.0 GRAMS PER LITER OF AMMONIATED GLYCYRRHIZIN, SAID SOLUTION HAVING A PH FROM ABOUT 2.0 TO 4.0; PROVIDING THE PLATING BY A PERIODIC CURRENT REVERSAL CYCLE ALTERNATING BETWEEN FORWARD CURRENT FOR ABOUT 4 TO 60 SECONDS AT A CATHODE CURRENT DENSITY OF ABOUT 50 TO 500 AMPERES PER SQUARE FOOT AND REVERSE CURRENT FOR ABOUT 1 TO 45 SECONDS AT A CATHODE CURRENT DENSITY OF ABOUT 30 TO 800 AMPERES PER SQUARE FOOT, WHILE MAINTAINING THE TEMPERATURE OF SAID SOLUTION BETWEEN ABOUT 90 TO 180* F. 